During starting or ramping up of a fossil-fired power plant, a boiler of the power plant, which is designed for producing steam, is initially operated at minimum load, which usually lies between 30% and 40% load. The live steam which is produced during this ramping-up phase is in this case customarily initially routed past the steam turbine directly to the condenser in so-called bypass mode. In the case of plants with a reheater, the live steam in this case is conducted via a high-pressure bypass station, spray-cooled to a low temperature level, and then directed into the cold line of the reheater. The steam which leaves the hot line of the reheater is conducted via an intermediate-pressure bypass station and cooled by means of spraying with water, and directed into the condenser. As a result of a high pressure level in the reheater, which customarily lies between about 20 bar and 30 bar, effective cooling of the reheater tubes, which are impinged upon by flue gas, is ensured.
For the operation of steam turbine plants, high demands are to be placed upon the purity of the steam. In particular, it is necessary to avoid particulate solids being entrained in the steam. Such solid body particles can lead to damage to the steam turbine and other plant components. Damage is incurred especially on the blading of the turbine.
In order to achieve slight superheating, it is necessary that the steam volume which is conducted via the intermediate-pressure bypass steam line is cooled to a low temperature level. This means that the bypass steam in the bypass steam line has to be spray-cooled with water. However, this requires a reliable and direct availability of the volume of water which is to be sprayed. Furthermore, a fast-acting spray water valve and a reliable measuring technique are required.
If the water which is to be sprayed is totally absent, precautions have to be taken so that no damage occurs. At present, the existing water spray protection devices are designed in such a way that only a total absence of spray water is taken into consideration.
Even temporarily falling short of the necessary volume of water is considered to be a malfunction, which in the worst case can lead to a total closing of the bypass steam line and could even lead to a trip, that is to say a shutdown of the entire power plant would be the consequence.
An initiated emergency shutdown of the bypass station can lead to repercussions upon the inspection interval or upon the service life of the gas turbine in the event of a gas turbine trip.
With the present precautions, it is quite possible that an emergency shutdown of the bypass station is carried out, even though a volume of water of more than 90% of the desired value has already been achieved and an emergency shutdown as such would not be necessary.